If everything is configured correctly I should only being using Quad9 as DNS Server, right? Then I was wondering how DNS-based Filtering actually works.
If it is forcing my DNS queries to whatever service I have configured, then what's the point for all the "custom" DNS you can configure?

dns filter global mode will force all devices to use the dns server you set, custom dns is for if you want to server a specific dns server that's not listed, no filtering will bypass the global dns filter if you set it to a device.

You are welcome I'm using pi hole for network based ad blocking, it's what I use for my DNS as a server that what my custom server is set to my rock64 use to be a raspberry pi, my 88u is not powerful enough to run some of the awesome scripts here that do the same thing.

great comments, i have been using the same, and i just set dns per device AIProtection dns filtering.
I have my printer and IOT devices using DNS home and use web and apps protection on these devices.
basically check all of these boxes and put them on 2.4 ghz and enable isolation mode and use 5 ghz for everything else.